At the electron accelerator ELSA a linearly polarised tagged photon beam is produced by coherent bremsstrahlung off a diamond crystal. Orientation and energy range of the linear polarisation can be deliberately chosen by accurate positioning of the crystal with a goniometer. The degree of polarisation is determined by the form of the scattered electron spectrum. Good agreement between experiment and expectations on basis of the experimental conditions is obtained. Polarisation degrees of P = 40% are typically achieved at half of the primary electron energy. The determination of P is confirmed by measuring the beam asymmetry, \Sigma, in pi^0 photoproduction and a comparison of the results to independent measurements using laser backscattering.
Beam asymmetry as a function of the PI0 centre of mass scattering angle.
Beam asymmetry as a function of the PI0 centre of mass scattering angle.
Beam asymmetry as a function of the PI0 centre of mass scattering angle.
We study the beam-energy and system-size dependence of \phi meson production (using the hadronic decay mode \phi -- K+K-) by comparing the new results from Cu+Cu collisions and previously reported Au+Au collisions at \sqrt{s_NN} = 62.4 and 200 GeV measured in the STAR experiment at RHIC. Data presented are from mid-rapidity (|y|<0.5) for 0.4 < pT < 5 GeV/c. At a given beam energy, the transverse momentum distributions for \phi mesons are observed to be similar in yield and shape for Cu+Cu and Au+Au colliding systems with similar average numbers of participating nucleons. The \phi meson yields in nucleus-nucleus collisions, normalised by the average number of participating nucleons, are found to be enhanced relative to those from p+p collisions with a different trend compared to strange baryons. The enhancement for \phi mesons is observed to be higher at \sqrt{s_NN} = 200 GeV compared to 62.4 GeV. These observations for the produced \phi(s\bar{s}) mesons clearly suggest that, at these collision energies, the source of enhancement of strange hadrons is related to the formation of a dense partonic medium in high energy nucleus-nucleus collisions and cannot be alone due to canonical suppression of their production in smaller systems.
Midrapidity $(|y| < 0.5)$ transverse momentum spectra of $\phi$ mesons for various collision centrality classes for $Cu+Cu$ collisions at $\sqrt{s_{NN}}=62.4$ and 200 GeV. To study the system size dependence, comparison of $40-50\%$ $Au+Au$ spectra to $10-20\%$ $Cu+Cu$ spectra at 200 GeV, and $40-60\%$ $Au+Au$ spectra to $20-30\%$ $Cu+Cu$ spectra at 62.4 GeV are shown. These centralities for the two colliding systems have similar $\langle N_{\scriptsize{\mbox{part}}}\rangle$ values as outlined in Table 2. The errors represent the statistical and systematic errors added in quadrature. They are found to be within the symbol size. The spectra are fitted to a Levy function discussed in the text.
Midrapidity $(|y| < 0.5)$ transverse momentum spectra of $\phi$ mesons for various collision centrality classes for $Cu+Cu$ collisions at $\sqrt{s_{NN}}=62.4$ and 200 GeV. To study the system size dependence, comparison of $40-50\%$ $Au+Au$ spectra to $10-20\%$ $Cu+Cu$ spectra at 200 GeV, and $40-60\%$ $Au+Au$ spectra to $20-30\%$ $Cu+Cu$ spectra at 62.4 GeV are shown. These centralities for the two colliding systems have similar $\langle N_{\scriptsize{\mbox{part}}}\rangle$ values as outlined in Table 2. The errors represent the statistical and systematic errors added in quadrature. They are found to be within the symbol size. The spectra are fitted to a Levy function discussed in the text.
Upper panels. $N_{\scriptsize{\mbox{part}}}$ scaled ($R^{N_{\scriptsize{\mbox{part}}}}_{AA}$) nuclear modification factors as a function of $p_{T}$ of $\phi$ mesons for $0-10\%$ and $20-30\%$ $Cu+Cu$ and $Au+Au$ collisions at $\sqrt{s_{NN}}=200$ GeV. Lower panel. Same as above for $N_{\scriptsize{\mbox{bin}}}$ scaled ($R^{N_{\scriptsize{\mbox{bin}}}}_{AA}$) nuclear modification factor. The error bars represent the statistical and systematic errors added in quadrature. The shaded band in upper panel around 1 at $p_{T}=4.5-5.5$ GeV/$c$ in the right side reflects the uncertainty in $N_{\scriptsize{\mbox{part}}}$ and that on the lower panel for $N_{\scriptsize{\mbox{bin}}}$ calculation for central $Au+Au$ collisions. The respective uncertainties for central $Cu+Cu$ collisions are of similar order.
The double helicity asymmetry in neutral pion production for p_T = 1 to 12 GeV/c has been measured with the PHENIX experiment in order to access the gluon spin contribution, Delta-G, to the proton spin. Measured asymmetries are consistent with zero, and at a theory scale of \mu^2 = 4 GeV^2 give Delta-G^[0.02,0.3] = 0.1 to 0.2, with a constraint of -0.7 < Delta-G^[0.02,0.3] < 0.5 at Delta-chi^2 = 9 (~3 sigma) for our sampled gluon momentum fraction (x) range, 0.02 to 0.3. The results are obtained using predictions for our measured asymmetries generated from four representative fits to polarized deep inelastic scattering data. We also consider the dependence of the Delta-G constraint on the choice of theoretical scale, a dominant uncertainty in these predictions.
Asymmetry in $\pi^0$ production as a function of $p_T$. Not included in the table: 8.3% common scale uncertainty (scales both the value and and the stat. uncertainty by the same factor), and 7x10$^{-4}$ common shift uncertainty.
$\pi^0$ asymmetry, combined Run-5 and Run-6
The PHENIX experiment presents results from the RHIC 2006 run with polarized proton collisions at sqrt(s) = 62.4 GeV for inclusive pi^0 production at mid-rapidity. Unpolarized cross section results are measured for transverse momenta p_T = 0.5 to 7 GeV/c. Next-to-leading order perturbative quantum chromodynamics calculations are compared with the data, and while the calculations are consistent with the measurements, next-to-leading logarithmic corrections improve the agreement. Double helicity asymmetries A_LL are presented for p_T = 1 to 4 GeV/c and probe the higher range of Bjorken_x of the gluon (x_g) with better statistical precision than our previous measurements at sqrt(s)=200 GeV. These measurements are sensitive to the gluon polarization in the proton for 0.06 < x_g < 0.4.
The fraction of inclusive $\pi^0$ yield which satisfied the BBC trigger condition.
The neutral pion production cross section at $\sqrt{s}$ = 62.4 GeV as a function of $p_T$ and the results of next-to-leading order (NLO) and next-to-leading logarithmic accuracy (NLL) perturbative Quantum Chromodynamics (pQCD) calculations for the theory scale $\mu$ = $p_T$.
The parameter $n$ obtained from the ratio of invariant cross section at $\sqrt{s}$ = 62.4 GeV and $\sqrt{s}$ = 200GeV, at each $x_T$ of $\sqrt{s}$ = 62.4 GeV data; error bars show the statistical and systematic uncertainties of the $\sqrt{s}$ = 62.4 GeV and $\sqrt{s}$ = 200 GeV data.
A measurement of the $\bjet$ production cross section is presented for events containing a $Z$ boson produced in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV, using data corresponding to an integrated luminosity of 2 fb$^{-1}$ collected by the CDF II detector at the Tevatron. $Z$ bosons are selected in the electron and muon decay modes. Jets are considered with transverse energy $E_T>20$ GeV and pseudorapidity $|\eta|<1.5$ and are identified as $\bjets$ using a secondary vertex algorithm. The ratio of the integrated $Z+\bjet$ cross section to the inclusive $Z$ production cross section is measured to be $3.32 \pm 0.53 {\rm (stat.)} \pm 0.42 {\rm (syst.)}\times 10^{-3}$. This ratio is also measured differentially in jet $E_T$, jet $\eta$, $Z$-boson transverse momentum, number of jets, and number of $\bjets$. The predictions from leading order Monte Carlo generators and next-to-leading-order QCD calculations are found to be consistent with the measurements within experimental and theoretical uncertainties.
Ratio of integrated Z0 + bjet cross section to inclusive Z0 production.
Ratio of the Z0 + bjet to Z0 cross section as a function of the bjet ET.
Ratio of the Z0 + bjet to Z0 cross section as a function of the bjet pseudorapidity.
We present a search for new particles whose decays produce two jets (dijets) using proton-antiproton collision data corresponding to an integrated luminosity of 1.13 fb-1 collected with the CDF II detector. The measured dijet mass spectrum is found to be consistent with next-to-leading-order perturbative QCD predictions, and no significant evidence of new particles is found. We set upper limits at the 95% confidence level on cross sections times the branching fraction for the production of new particles decaying into dijets with both jets having a rapidity magnitude |y| < 1. These limits are used to determine the mass exclusions for the excited quark, axigluon, flavor-universal coloron, E6 diquark, color-octet technirho, W', and Z'.
The measured dijet mass spectrum for both jets having rapidity from -1 to 1.
Measurements of the kinematic distributions of $J/\psi$ mesons produced in $p-$C, $p-$Ti and $p-$W collisions at $\sqrt{s}=41.6 \mathrm{GeV}$ in the Feynman-$x$ region $-0.34 < x_{F} < 0.14$ and for transverse momentum up to $p_T = 5.4 \mathrm{GeV}/c$ are presented. The $x_F$ and $p_T$ dependencies of the nuclear suppression parameter, $\alpha$, are also given. The results are based on $2.4 \cdot 10^{5}$ $J/\psi$ mesons in both the $e^+ e^-$ and $\mu^{+}\mu^{-}$ decay channels. The data have been collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The measurement explores the negative region of $x_{F}$ for the first time. The average value of $\alpha$ in the measured $x_{F}$ region is $0.981 \pm 0.015$. The data suggest that the strong nuclear suppression of $J/\psi$ production previously observed at high $x_F$ turns into an enhancement at negative $x_F$.
J/PSI distributions (dN/dp_T, normalized to their integrals over the measured range) for three targets materials with statistical and systematics uncertainties.
J/PSI distributions (dN/dx_F, normalized to their integrals over the measured range) for three targets materials with statistical and systematics uncertainties.
The parameter Alpha as a function of p_T. Statistical and systematic uncertainties are indicated separately.
Deeply virtual Compton scattering has been measured in e^+p collisions at HERA with the ZEUS detector using an integrated luminosity of 61.1 pb^-1. Cross sections are presented as a function of the photon virtuality, Q^2, and photon-proton centre-of-mass energy, W, for a wide region of the phase space, Q^2>~1.5 GeV^2 and 40<W<170 GeV. A subsample of events in which the scattered proton is measured in the leading proton spectrometer, corresponding to an integrated luminosity of 31.3 pb^-1, is used for the first direct measurement of the differential cross section as a function of t, where t is the square of the four-momentum transfer at the proton vertex.
The DVCS cross section as a function of Q**2.
The DVCS cross section as a function of W.
The DVCS cross section as a function of W in four Q**2 regions.
The dissociation of virtual photons, $\gamma^{\star} p \to X p$, in events with a large rapidity gap between $X$ and the outgoing proton, as well as in events in which the leading proton was directly measured, has been studied with the ZEUS detector at HERA. The data cover photon virtualities $Q^2>2$ GeV$^2$ and $\gamma^{\star} p$ centre-of-mass energies $40<W<240$ GeV, with $M_X>2$ GeV, where $M_X$ is the mass of the hadronic final state, $X$. Leading protons were detected in the ZEUS leading proton spectrometer. The cross section is presented as a function of $t$, the squared four-momentum transfer at the proton vertex and $\Phi$, the azimuthal angle between the positron scattering plane and the proton scattering plane. It is also shown as a function of $Q^2$ and $\xpom$, the fraction of the proton's momentum carried by the diffractive exchange, as well as $\beta$, the Bjorken variable defined with respect to the diffractive exchange.
The differential cross section DSIG/DT for the LRG and the LPS data samples.
The fitted exponential slope of the T distribution as a function of X(NAME=POMERON).
The fitted exponential slope of the T distribution as a function of X(NAME=POMERON).
The semi-inclusive reaction e+ p -> e+ X p was studied with the ZEUS detector at HERA using an integrated luminosity of 12.8 pb-1. The final-state proton, which was detected with the ZEUS leading proton spectrometer, carried a large fraction of the incoming proton energy, xL>0.32, and its transverse momentum squared satisfied pT^2<0.5 GeV^2/ the exchanged photon virtuality, Q^2, was greater than 3 GeV^2 and the range of the masses of the photon-proton system was 45
Double differential cross sections as a funtion of PT**2 for the XL range 0.32 TO 0.38. The methods S123 and S456 are the results using different stations of the silicon microstrip detectors.
Double differential cross sections as a funtion of PT**2 for the XL range 0.38 TO 0.44. The methods S123 and S456 are the results using different stations of the silicon microstrip detectors.
Double differential cross sections as a funtion of PT**2 for the XL range 0.44 TO 0.50. The methods S123 and S456 are the results using different stations of the silicon microstrip detectors.
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